The present invention relates to the production and processing of high T.sub.c superconducting bismuth-strontium-calcium-copper-oxide materials.
Since the discovery of the copper oxide ceramic superconductors, their physical and chemical properties have been widely studied and described in many publications, too numerous to be listed individually. These materials have superconducting transition temperatures (T.sub.c) greater than the boiling temperature (77.degree. K) of liquid nitrogen. However, in order to be useful for the majority of applications, substantially single phase superconducting materials with high critical current densities (J.sub.c) are needed. In general, this requires that the grains of the superconductor be crystallographically aligned, or textured, and well sintered together. Several members of the bismuthstrontium-calcium-copper-oxide family (BSCCO), in particular, Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 (BSCCO 2212) and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (BSCCO 2223) have yielded promising results, particularly when the bismuth is partially substituted by dopants, such as lead ((Bi,Pb)SCCO).
Composites of superconducting materials and metals are often used to obtain better mechanical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by the well-known "powder-in-tube" or "PIT" process which includes, for multifilamentary articles, the three stages of: forming a powder of superconductor precursor material (precursor powder formation stage); filling a noble metal billet with the precursor powder, longitudinally deforming and annealing it, forming a bundle of billets or of previously formed bundles, and longitudinally deforming and annealing the bundle to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material in a surrounding noble metal matrix (composite precursor fabrication stage); and subjecting the composite to successive asymmetric deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing stage). General information about the PIT method described above and processing of the oxide superconductors is provided by Sandhage et al., in JOM, Vol. 43, No. 3 (1991) pages 21 through 25, and the references cited therein, by Tenbrink, Wilhelm, Heine and Krauth, Development of Technical High-Tc Superconductor Wires and Tapes, Paper MF-1, Applied Superconductivity Conference, Chicago (Aug. 23-28, 1992), and Motowidlo, Galinski, Hoehn, Jr. and by Haldar, Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors, paper presented at Materials research Society Meeting, Apr. 12-15, 1993.
In the composite precursor fabrication stage, longitudinal deformation operations, i.e., wire drawing and/or extrusion, which form the billet or bundle into an elongated shape such as a wire or tape are followed by low temperature anneals, typically on the order of 200 C. to 450 C. at 1 atm in air for silver, to relieve strain energy introduced by deformation, without causing substantial reaction of the precursor powder or melting or grain growth in the silver. FIG. 1 (prior art) is a typical annealing curve showing silver hardness as a function of annealing temperature. In some instances, a high temperature thermal anneal, typically on the order of 600 C. at 1 atm in air for silver, has been performed prior to the first bundle deformation step in the stage to bond the billets to one another. In other instances, where high strain deformations involving reductions of 100% or more have been performed, a high temperature thermal anneal, typically on the order of 600 C. at 1 atm in air for silver, has been included as the last step in the stage in order to relieve the strain energy in the matrix material prior to thermomechanical processing.
The deformation portions of the deformation and annealing cycles in the thermomechanical processing stage, are asymmetric deformations which create alignment of precursor grains in the core ("textured" grains) which facilitate the growth of well-aligned and sintered grains of the desired superconducting material during later thermal processing stages. Examples are rolling and the isostatic pressing cycle described in U.S. patent application Ser. No. 07/906,843 (U.S. '843) filed Jun. 30, 1992 entitled "High Tc Superconductor and Method for Making It", which is herein incorporated in its entirety by reference. They may be followed by anneals to relieve strain energy in the metal portions of the composite precursor. A series of heat treatments is also typically performed during the thermomechanical processing stage to promote powder reactions, including final thermomechanical treatment stages employed to more fully convert the filaments to the desired final, highly textured superconducting phase, preferably BSCCO or (Bi, Pb)SCCO 2223. The thermomechanical processing may be carried out by any conventional method, such as for example those described in Sandhage et al, supra, Tenbrink et al, supra, Haldar, supra, and in U.S. Pat. No. 5,635,456 issued Jun. 3, 1997, entitled "Improved Processing for Oxide Superconductors," and U.S. Pat. No. 5,661,114 issued Aug. 26, 1997, entitled "Improved Processing of Oxide Superconductors", and U.S. patent application Ser. No. 08/468,089, (U.S. '089) filed Jun. 6, 1995 entitled "Improved Deformation Process for Superconducting Ceramic Composite Conductors", and Ser. No. 08/651,169 (U.S. '169) filed May 21, 1996, entitled "A Novel reaction for High Performance (Bi, Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y Composites", all of which are hereby incorporated in their entirety by reference.
The general process is practiced in several variants depending on the starting powders, which may be, for example, metal alloys having the same metal content as the desired superconducting core material in the "metallic precursor" or "MPIT" process, or mixtures of powders of the oxide components of the desired superconducting oxide core material or of a powder having the nominal composition of the desired superconducting oxide core material in the "oxide powder" or "OPIT" process. OPIT precursor powders may be prepared by reacting raw powders such as the corresponding oxides, oxalates, carbonates, nitrides or nitrates of the metallic elements of the desired superconducting oxide. One or more subsequent chemical reactions, some of which typically occur inside the formed filaments, create the superconducting material in combination with greater or lesser amounts of non-superconducting secondary phases. Because the desired superconducting material is formed by a series of chemical reactions, its performance will depend on the quality and chemical composition of the starting materials and on the subsequent processing conditions, such as temperature, time, and atmosphere. Different processing conditions will give rise to different phases or different ratios of phases, some of which, being easier to mechanically texture or more likely to achieve complete reaction into the final superconducting material, are more desirable than others. Various intermediate reactions may be deliberately promoted in order to create more desirable intermediate phases or to increase the ratio of the final superconducting material to the secondary phases in the finished product.
For example, it has been observed that the orthorhombic phase of BSCCO 2212 responds better to the asymmetric deformation required for deformation-induced texturing resulting in a denser, less porous oxide grain structure, and so, undergoes texturing to a much greater extent than the corresponding tetragonal phase. Moreover, the orthorhombic phase of (Bi,Pb)SCCO 2212 represents doping of lead into the BSCCO solid state structure with the concomitant conversion of the lead-free tetragonal phase into the orthorhombic phase. The lead-doped orthorhombic phase readily converts to the final superconductor, (Bi,Pb)SCCO 2223 to give a high quality superconductor over a large temperature range. In comparison, the lead-free tetragonal BSCCO phase does not convert readily into (Bi,Pb)SCCO 2223. By controlling phase conversions, it is possible to make use of the advantages of the orthorhombic and tetragonal phases, by using the particular phase most suited to the operation to be performed. Methods of controlling the phase composition of the precursor powder during its preparation and during subsequent thermomechanical processing, are described, for example in U.S. patent application Ser. No. 08/467,033 (U.S. '033) filed Jun. 6, 1995 and entitled "Processing of (Bi,Pb)SCCO Superconductor in Tapes and Wires", which is herein incorporated in its entirety by reference. In the process described in U.S. '033, an elongated BSCCO superconducting article is manufactured by first heating a mixture of raw materials of a desired ratio of constituent metallic elements corresponding to a final superconducting BSCCO material at a first selected processing temperature in an inert atmosphere with a first selected oxygen partial pressure for a first selected time period. The first processing temperature and partial pressure are cooperatively selected to form a dominant amount of certain desired BSCCO precursor phases, preferably including a tetragonal BSCCO 2212 phase, along with the secondary phases necessary for the production of the desired final superconducting phases, in the reacted mixture. A composite article may be formed using this reacted precursor powder substantially surrounded by a constraining metal matrix. Prior to the texture-inducing deformation operation, the article is subjected to a heat treatment at a second selected processing temperature in an inert atmosphere with a second selected oxygen partial pressure for a second selected time period which favors conversion of the tetragonal BSCCO phase into the corresponding orthorhombic BSCCO 2212 phase, so as to form a dominant amount of an orthorhombic BSCCO 2212 phase in the reacted mixture. Thereafter, the multifilamentary article is textured by deformation and thermally processed into a BSCCO 2223 oxide superconductor article. Selection of appropriate processing conditions, for example as described in Luo et al., "Kinetics and Mechanism of the (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 Formation Reaction in Silver-Sheathed Wires," Applied Superconductivity, Vol. 1, No. 1/2, pp. 101-107 (1993), will allow the BSCCO 2223 to substantially inherit the texture, whether orthorhombic or tetragonal, of its 2212 precursor phase.
Reference to the "orthorhombic phase" and the "tetragonal phase" recognizes the existence of two crystallographic structures for BSCCO superconducting materials, the tetragonal and the orthorhombic structures. The tetragonal structure has equivalent a and b axes with a lattice parameter of about 5.4 angstroms. The conversion of the tetragonal to the orthorhombic structure corresponds to the formation of an oxygen deficient structure with a and b axes which are unequal in length. See, Jeremie et al in Supercond. Sci. Technol. 6 (1993) pages 730 through 735, which is herein incorporated by reference in its entirety. The conversion occurs simultaneously with the incorporation of a substituent having a variable oxidation state, i.e., Pb or Sb, into the BSCCO structure. Thus the formation of the orthorhombic phase is indicative of the reaction of the dopant carrier. The conversion is indicated by the broadening (and under some conditions, complete splitting) of the XRD 200 and 020 peaks at 33.degree. (2.theta.).
As compared to certain prior art approaches, this process provides a method for preparing precursor powders having a controlled phase composition in a single step reaction process, and improved phase control during subsequent thermomechanical processing. However, it has been found that when the tetragonal to orthorhombic phase conversion is performed in multifilamentary composite precursors, processing inhomogeneities tend to occur and blister-like defects frequently form, both of which can adversely affect the J.sub.c performance of the desired superconducting composite article. The inventors believe that during the composite precursor fabrication stage, the mechanical force applied to reduce the cross-section of the multifilamentary precursor will tend to work to a greater degree on the filaments in the outer portions of the precursor and cause an inhomogeneous stress distribution, both through the diameter of the precursor and along its length. Therefore, the outer filaments and their surrounding matrix material will deform more than those near the center of the precursor, creating a distribution of differently sized filaments. Further, the inhomogeneous stress distribution creates filament slippages, breaks and other defects in the filaments. During the composite precursor fabrication stage, the multifilamentary precursor also tends to absorb gas and moisture which becomes trapped, creating blisters, particularly in the filaments and at the interfaces between the filaments and the surrounding metal matrix. These problems are characteristic of PIT processes generally, but they are exacerbated during processes requiring high temperature treatments and oxygen release, such as the tetragonal to orthorhombic phase transformation. Significant amounts of oxygen must be released from the filaments during the formation of the oxygen-deficient structure which characterizes the orthorhombic phase, and removed by diffusion through the matrix material. If the cross-sections of the filaments and surrounding matrix material are non-uniform, the phase transformation cannot proceed uniformly and undesired phases will result. The positive pressure inside the blisters will tend to prevent oxygen release from the adjacent filaments causing additional inhomogeneities in the phase transformations. Moreover, during the high temperature phase conversion, the gas in the blisters will tend to expand and water and other condensed phases will volatize so the blisters will grow significantly in size, hindering subsequent processing steps.
It is desirable to provide a process which provides improved powder phase control coupled with improved oxygen control and defect management during tetragonal to orthorhombic phase conversions. It is also desirable to provide a superconducting composite article with reduced defect levels and improved J.sub.c performance.